In case you missed it - Successfully raising the formation! The Hinksey Flood Alleviation scheme

Rail travellers will be aware that flooding in the Hinksey area, on the route between Didcot and Oxford, has been a long-standing problem that has had a severe impact on the local community which relies on the railway. In the last 15 years, the route has had to be closed more than 11 times, causing extensive disruption to passengers and businesses through the cancellation and diversion of passenger and freight services.

The flooding of the railway is part of a much bigger flood problem that is being addressed by the Environment Agency as part of the Oxford Flood Alleviation Scheme. The area around the railway at Hinksey has been often described as a “bathtub”, with floodwater accumulating around and over the railway formation as it tries to find an easy path to the river Thames. The railway formation is acting as a barrier and generations of gravel extraction, followed by major land fill developments in the 1970s, has helped to create a pinch point for flood water at Hinksey.

Raising the railway

To address this problem, Network Rail developed its own Hinksey Flood Alleviation scheme. Joanna Grew, the Network Rail commercial scheme sponsor and Edward John, who is Network Rail’s scheme project manager, explained the work being undertaken.

“During the past two years, as part of our Railway Upgrade Plan to deliver a better railway for passengers, we have worked with the Environment Agency to find a long-term solution to the problem,” Joanne Grew stated. “This has involved carrying out detailed flood modelling and an in-depth environmental study to establish the cause of the flooding and the options available to reduce the chances of it happening again.”

The £21 million scheme involved raising the railway by approximately 650mm over a length of about 400 metres. This will bring the track above the maximum recorded flood levels along this section of railway. That doesn’t sound too challenging, but when it includes an overbridge and a series of underbridges, as well as four tracks – two of them 90 mph main lines – plus nine S&C units and the installation of a twin-box culvert 80 metres long, it gets a bit more interesting. Then, of course, there are the stray wallabies to contend with, but more about that later.

To enable this work to be completed before the winter sets in, a blockade was organised for 16 days, starting on 30 July lasting until Monday 15 August.

One of the consequences of raising the track levels, one that was identified at an early stage, was that the railway formation would, when raised, act as a dam across the floodplain. This means that there would be a potential increase in flooding to the west of Hinksey, an area which includes important link roads as well as residential properties.

Removing the dam effect

To address this potential problem, Network Rail procured the services of design consultant AECOM to develop an Environmental Statement for the Hinksey Flood Alleviation scheme. Detail flood modelling of the area concluded that the optimum solution was to install a culvert under the railway formation, removing the dam effect and ensuring that the scheme did not have a detrimental effect on the surrounding area.

Preparatory work started way back in 2012, when Network Rail replaced the overbridge at Abingdon Road, which crosses the railway right in the middle of the proposed track lift. At the time, when this work was planned, the main reason for the reconstruction was to improve gauge clearances for freight traffic and to prepare for electrification. Incorporating the need to accommodate the proposed 650mm track lift did not require major changes to the original scheme.

Further work started in February this year to prepare the area for the additional infrastructure improvements that included the repair and reconstruction of Stroud’s underbridges and the construction of the new culvert as well as the track lift itself. The work also included preparing the track and replacing signals, as well as setting up a site compound with welfare facilities at the Redbridge Park and Ride. This work was planned every Sunday right through to 17 July, in preparation for the main engineering work that was carried out during the 16 day blockade mentioned earlier.

Carillion was the principal contractor for the refurbishment of the civils work – the Stroud bridges and culvert installation – while, as part of its S&C Alliance with Network Rail, Colas Rail was the principal contractor for the track lift. Two laydown areas were created closer to the site on the east side of the railway line, and dewatering pumping systems in conjunction with a silt removal process set-up to allow the installation of the culverts to be undertaken. This work took place in preparation for the 16-day closure of the line between Didcot and Oxford stations, which started on 30 July.

The first five days were focussed on removing the old track and S&C, then carrying out repairs and replacement of parts of Stroud’s underbridge, which is located 100 metres to the south of the Abingdon Road overbridge previously referred to, and the construction of the new culvert which is located to the north of the overbridge.

Bridge jacking

The proposals prepared by AECOM included the replacement of parts of the four separate bridge decks that together form Stroud’s underbridge. This work included the retention of the two main line steel decks, jacking them up to accommodate the track lift, installing 220mm deep concrete bearing beams, and repairing and refurbishing the eight bearing pedestals. As the freight line deck was life expired, it was replaced by a Z-type steel deck structure. The fourth deck, which was in fact redundant, was replaced with a steel footbridge designed to carry signalling, telecommunication and power cables for the route.

Whilst this work was in progress, the new culvert was being installed across the railway formation. A total of 64 reinforced-concrete box units, each 2.5 metres long, 2.4 metres high, 4.2 metres wide and weighing 25 tonnes, had been delivered to site from Ireland, manufactured by Banagher Concrete.

The plan, which was successfully carried out, was to install 44 of the units during the possession, knowing that the rest could be installed with the route open. Using a 300 tonne crawler crane, operating from two crane pads, a consolidated base was prepared. Then the pre-cast units were positioned to form two box culverts side by side, approximately 80 metres in length.

Early handback

Following the success of the first five days of the blockade, Carillion handed over the mantle of principal contractor to Colas twelve hours early.

Using a Kirow 1200 crane, working south to north, Colas removed the remaining track, scarified the existing ballast and prepared the new formation, which included a geotextile blanket.

Once a waterproofing system had been installed on the Stroud bridge structures, 1,200 metres of plain line track was constructed along with nine new S&C units including one set of traps. Because of the hot weather, stressing and welding had to take place at night. The blockade was handed back three hours early, after completing all the stressing plus 200 welds, with a temporary speed restriction of 50mph.

Edward John was keen to point out that an integral part of the success of this work was the very effective way that the two principal contractors had worked together throughout the blockade. They carried out regular interface meetings which were “incredibly effective” in ensuring that the essential work during the blockade was completed.

Wallabies!

There were some sensitive environmental matters that had to be handled throughout the work. The works were carried-out under Section 61 consents and, with a campsite right next door, noise mitigation was paramount. All works within the local watercourses required Environmental Agency Consent.

Grass snakes, being excellent swimmers, appeared everywhere so they had to be relocated. A crow’s nest required a significant rethink to recapture the significant delays it created, and the problem of a nesting moorhen was resolved by Mother Nature in the form of a hungry stoat. Oh! And one must not forget the wallabies that apparently exist in abundance in Oxfordshire and, on occasion, enjoy a bit of train spotting!

There is still a fair amount of work to do – raising S&T location boxes throughout the area, constructing the culvert wing/head walls and installing gabions to help support the raised embankment. The channels and ditches need to be cleared of vegetation to improve water flows and scour protection is required locally at the culvert headwalls, requiring complete damming of the watercourse.

In addition, throughout the work, consideration has had to be given to the depth of digs and disturbance because the area is a site of medieval archaeological interest.

The culverts are designed to carry a water flow that forms part of the completed Oxford Flood Alleviation Scheme. However, at present, the Oxford scheme is not yet developed enough to cater for the anticipated increase in water flow. So, at the outlet side of the culverts, Network Rail is having to install ‘orifice plates’ which are designed to restrict the outflow until the Oxford scheme is able to cater for the increased flow of water.